Abstract: The present disclosure relates to a topical pharmaceutical composition containing N-[(1S)-1-(5-fluoropyrimidin-2-yl)ethyl]-3-(5-isopropoxy-1H-pyrazol-3-yl)-3H-imidazo[4,5-b]pyridin-5-amine (hereinafter also referred to as “Compound A”), or a pharmaceutically acceptable salt thereof; to the use of the topical pharmaceutical composition as a medicament; to processes for the preparation of said topical pharmaceutical composition; to certain new methods of treating an inflammatory skin disorder, particularly psoriasis, by administering a topical pharmaceutical composition containing Compound A or a pharmaceutically acceptable salt thereof; and to novel crystalline forms of a mesylate salt of Compound A.